1. Technical Field
This invention relates to integrated circuit manufacture and, in particular, manufacture of integrated circuits having shallow junctions.
2. Art Background
In the manufacture of integrated circuits, electrical contact to junctions such as the source and drain of field effect transistors is required. For these devices desirable electrical properties include a junction contact resistance less than 10 ohms and a junction leakage current less than 10.sup.-7 amps/cm.sup.2. Complexities inherent in strict design rules make satisfaction of these requirements significantly more difficult. (The device design rule is the smallest lateral dimension for all features within the device circuit.) For example, since the source and drain junctions are typically no deeper than 0.25 .mu.m at submicron design rules, any undesirable chemical reaction with the junction induced by the fabrication process quickly destroys it. Additionally, electrical contact is made through an opening in an overlying electrical insulator material, also known as a dielectric, to the underlying junction region, e.g., drain or source junction region (an electrical insulator material is defined in C. Kittel, "Introduction to Solid State Physics, 3rd Ed." p. 252 and p. 266 (1967)). Typically, as a consequence of strict design rules, this opening (window) has a high aspect ratio, i.e., greater than 1.1. (Aspect ratio is defined as the thickness of the dielectric at the junction divided by the effective diameter of the window at the junction, i.e., the diameter of a circle having the same area as the window at the junction.) To contact the junction through a high aspect ratio opening requires deposition of a conductive material that conforms to or fills the opening so that the conducting cross-section in the window is adequate to maintain an acceptably low current density. Additionally, the same deposition that produces the contact through the window is employed to form a surface metallic layer on the dielectric, and this surface layer is ultimately patterned to form electrical interconnects between contacts. The surface layer, as a result, should not have defects that unacceptably degrade the electrical characteristic of the interconnect. Thus, in summary, to ensure a suitable contact and attendant interconnect, undesirable chemical reaction with the junction should be avoided while a coating that conforms to or fills the window should be produced and while an adequate surface deposition is obtained.
Generally, it is desirable to utilize aluminum as the electrically conductive contact and interconnect material because of its high conductivity, etchability, excellent adhesion to silicon oxide, and nominal cost. However, direct deposition of aluminum to produce a suitable coating in a window that has an aspect ratio greater than 1.1 has been an elusive goal. One relatively recent procedure involving the chemical vapor deposition using a triisobutyl aluminum precursor (see H. W. Piekaar, et al., Sixth International IEEE VLSI Multilevel Interconnect Conference, p. 122 (1989), L. F. Kwakman, et. al. Tungsten and Other Refractory Metals for VLSI Applications IV, p. 315, MRS (1989)) for depositing an aluminum contact in these demanding applications shows promise and is undergoing further investigation. However, since triisobutyl aluminum does not dissociate spontaneously on a dielectric material a chlorine compound, titanium tetrachloride, is used to initiate growth of the aluminum film. This procedure leaves chlorine residue at the aluminum-dielectric interface which tends to diffuse into the aluminum interconnect conductors and to cause corrosion of the conductor lines, with the resulting failure of the electronic circuit. Thus, approaches for aluminum contacts are either totally unacceptable or have some reliability considerations.
Other contact materials have been investigated. For example, the deposition of tungsten by a low pressure chemical vapor deposition (LPCVD) technique has been reported. (See, for example, N. E. Miller and I. Beinglass, Solid State Technol., 25(12), 85 (1982), E. K. Broadbent and C. L. Ramiller, J. Electrochem. Soc., 131, 1427 (1984), and E. K. Broadbent and W. T. Stacy, Solid State Technol., 49(12), 51 (1985).) This technique has the advantage of allowing deposition into windows that have aspect ratios greater than 1.1. However, there is a substantial undesirable interaction between the junction (including a silicide overlying the silicon) and chemical entities introduced during deposition of the tungsten. (See, for example, M. L. Green and R. A. Levy, Semicon East 1985 Technical Proceedings, 57 (1985), M. L. Green and R. A. Levy, J. Electrochem. Soc., 132, 1243 (1985), G. E. Georgiou, et al., Tungsten and Other Refractory Metals for VLSI Applications II, E. K. Broadbent, Editor, page 225, MRS, Pittsburgh, PA, 1987, and N. Lifshitz, et al. Tungsten and Other Refractory Metals for VLSI Applications III, V. A. Wells, Editor, page 225, MRS. Pittsburgh PA, 1988.) Under certain conditions this interaction is self-limiting. That is, the amount of junction silicon consumed during the deposition reaches a maximum value that depends on the reaction conditions. Despite this self-limiting effect, the damage produced is still too extensive for junctions, such as source and drain junctions, shallower than 0.25 .mu.m. Additionally, the adhesion between the deposited tungsten material and the dielectric material, e.g., silicon dioxide, is not entirely desirable. One proposal involving tungsten deposition (N. Lifshitz and R. Shultz, U.S patent application Ser. No. 07/226,917, filed Aug. 1, 1988) has produced a suitable contact and interconnect deposition. This procedure is quite suitable for demanding applications but involves somewhat complicated processing. Thus direct, acceptable deposition of aluminum is still an alluring possibility.